Fluorescence detecting apparatus is commonly used in the fields of analytical chemistry and cytometry, and for those applications, methods of calibration are well developed and well-known to those skilled in those fields. For example U.S. Pat. Nos. 5,093,234 and 4,868,126 and many other patents and publications disclose methods of calibrating flow cytometers and fluorescence microscopes. The apparatus and method of the present invention is not particularly suited to these fields of scientific analysis.
The field of application of the present invention relates particularly to the uses of fluorescence detection in the authentication of genuine articles and in the detection of counterfeit articles, and to the uses of similar apparatus in process control and quality control.
The counterfeiting of documents such as passports, customs and immigration documents, identification cards, driver's licenses, and credit cards has become a problem that is costly to the public in many ways. When such documents are used by unauthorized persons, such use often results in direct financial losses to the public and in indirect losses due to increased cost of law enforcement and increased cost of insurance against losses. Similar problems and costs to the public occur as a result of counterfeiting or "pirating" of brandname products by unauthorized manufacturers and by copyright infringers.
Some of the systems that have been developed to prevent these problems have used marking with fluorescent substances to identify authentic articles, and such systems have used non-visible radiation to excite the fluorescent substances to confirm the identity of authentic articles or to reveal counterfeit articles which lack the predetermined fluorescent markings. Some systems using fluorescent markings depend on quantitative measurements of quantifiable characteristics of the fluorescent radiation, such as the intensity, wavelength or spatial distribution of the fluorescent markings. An example of such a system is disclosed in U.S. Pat. No. 4,642,526 (Hopkins, 1987). It is these systems and similar systems described below to which the present invention is directed.
The same apparatus that is used for authentication of articles can also be used in quality control and in process control of manufacturing processes. In those applications, the presence or absence of a required substance or the amount of such a substance is determined by tagging the required substance with a fluorescent substance and by using a fluorescence detector to measure the tagged combination. The calibration of systems used for these purposes can also be accomplished easily with the present invention.